The invention relates to a filter, and more particularly especially a coffee filter, and to a method for producing a coffee filter.
It is known to fill infusion bags with tea and also ground coffee, the filling quantity being intended for one or more cups. However, after brewing, coffee infusion bags result in an unsatisfactory drink because the extract content is too low, or the bags have to be agitated intensively in the cup for a long enough time to achieve the necessary extract content. Furthermore, the ground coffee absorbs a considerable quantity of water and as a result, the cup is no longer full after the bag has been taken out.
Starting from this state of the art, the applicant has already proposed providing a filter, especially a coffee filter, which is characterized by a holding frame which can be placed on the edge of a cup or such like preparation vessel and which has vertical or approximately vertical walls, and by a bag fastened inside this frame and consisting of filter material, especially filter paper, the filter bag being fastened to the holding frame along the entire inner peripheral edge of the frame, so that the entire free cross-section of the holding frame is filled by the filter bag. Preferably, the holding frame is triangular, as seen in horizontal projection, the walls being produced from a paperboard or cardboard blank. In the last-mentioned embodiment, the cardboard blank which can be unfolded into a triangle is foldable inwards, preferably on at least one side, to form an M-fold with the respective adjoining side walls of the cardboard blank forming the triangular frame.
The significance of the proposal mentioned is, therefore, that a cup filter distinguished by being highly functional and easy to handle is provided.
It will now be shown that, when a filter of this type is used, the filter bag is pressed in the upper region against the holding frame or its vertical walls by the brewing water. At the same time, the infusion flowing through the nonwoven material endeavours to run off along both the filter bag and the holding frame or its walls. However, contact between the coffee infusion and the cardboard is undesirable for reasons of acceptability. Furthermore, there is the danger that some of the coffee will flow along the holding frame or its walls next to the cup or the like.
The object of the present invention is, therefore, to provide a filter of the type mentioned, in which the infusion or coffee runs off into the cup or the like along the filter bag only. This object is achieved through the use of a filter according to the present invention and through the method for producing such a filter;
The measures according to the invention ensure that the infusion flowing through the filter bag runs off into the cup or such like vessel along the filter bag only, and conventional grades of cardboard or paperboard can be used for producing the holding frame.
According to the invention, therefore, it is necessary to ensure that the holding frame is made hydrophobic, that is to say water-repellent, on the side facing the filter bag, whereas the filter bag itself is to consist of a hydrophilic filter material. The filter bag must therefore have good wettability with water, so that the infusion runs off into the cup solely along the filter bag. Conventional nonwoven filter materials with a non-oriented fibre arrangement and a comparatively smooth surface are unsuitable for this; despite the use of a hydrophobised holding frame, the infusion for the most part runs off along this and past the cup.
According to the invention, the aim sought after, as regards the design of the filter bag, is achieved in an especially simple way in that the nonwoven filter material has a fibre arrangement which is oriented at least partially, but preferably predominantly, in the direction of flow of the infusion, and/or in that the nonwoven filter material has a surface of a textile-like structure. The last-mentioned design and the resulting roughness of the surface structure ensure a certain "adhesion" of the liquid film on the filter bag which assists the run-off along the filter bag.
Nonwoven materials suitable for this have a weight per unit area of between 16 and 17 g/m.sup.2, preferably approximately 16.5 g/m.sup.2, and a thickness of between 0.05 and 0.07 mm, preferably approximately 0.056 mm.
Especially good results, also in terms of processibility, are obtained if the nonwoven materials consist of a natural-fibre base layer, onto which fusiable plastic fibres are coated as a heat-sealable layer.
The holding frame or its vertical walls preferably consist of cardboard or paperboard, the side facing the filter bag being hydrophobised in such a way that, when wetted with a drop of water, a wetting angle of &gt;100.degree., especially &gt;105.degree., is obtained.
In order to make the paperboard hydrophobic in this way, the appropriate size is treated with means which react chemically with the functional hydroxy groups of cellulose, especially with a mixture of dimeric alkylketene, water and ammonium zirconium carbonate.
In order to make the paperboard hydrophobic, it can also be treated with means which, in a chemical process, precipitate on the fibre to form a water-insoluble water-repellent compound, especially with an aqueous sodium stearate and aluminium acetate solution.
An especially preferred process for hydrophobising the paperboard or the paperboard blank for producing a holding frame for a filter of the type described is characterized in that a mixture of
68 ml of 20% aqueous dispersions of a dimeric alkylketene in water,
910 ml of water,
22 ml of a 20% ammonium zirconium carbonate solution dissolved in water in quantities of approximately 20 to 50 g/m.sup.2 is coated onto the side of the paperboard to be hydrophobised and is dried at approximately 120.degree. C.
Another process for hydrophobising the paperboard is characterized in that approximately 30 g/m.sup.2 of a 1% aqueous sodium stearate solution are coated onto the paperboard, in order then to apply approximately 30 g/m.sup.2 of a 1.5% aluminium acetate solution to the still moist paperboard, and in that the paperboard so treated is subsequently dried at approximately 60.degree. C.
A wetting angle of 120.degree. is obtained as a result of the first-mentioned process; a wetting angle of 124.degree. is obtained as a result of the second-mentioned process.
Tests were conducted with a paperboard having a a weight of approximately 180 and 240 g/m.sup.2, a gluing coefficient in the range of between approximately 1.5 and 15 and a porosity of between approximately 15 and 250 Gurleys per second.
It was found, furthermore, that hydrophobising sufficient even under contact with hot water can be achieved only if the paperboard used meets certain requirements. Thus, the latter should not contain even traces of any substances with the character of a wetting agent as a result of its production process, since these would counteract the hydrophobic effect.
The said hydrophobising agents can also be added during the production of the paperboard from paper pulp.
It is necessary to ensure, at all events, that the hydrophobising agent is anchored so firmly to the cellulose fibre of the fibrous material that it cannot be washed off by the hot infusion.
To prevent the paperboard holding frame from being made hydrophobic, the present invention provides an alternative which is characterized in that a hydrophobically active skirt is arranged in the upper region between the filter bag on the one hand and the holding frame or its walls on the other hand. This skirt preferably consists of a thin film, for example polyethylene film, which is incorporated and welded in during the production of the filter. Because this skirt is arranged in the upper region between the filter bag and the holding frame, the water poured in can flow off downwards, without reaching the holding frame or its paperboard walls.
Moreover, the thickness of the paperboard walls of the holding frame is between approximately 0.2 and 0.4, preferably approximately 0.3 mm.
As already stated above, to obtain a high degree of hydrophobisation it is important that the initial paper-board should not contain any substances with the character of a wetting agent as a result of its production, since these would counteract the hydrophobic effect. It is possible to check this by means of a measurement of the wetting angle; the wetting angle of the untreated paperboard should be more than 80.degree..